As is well known, the error rate in decoding a received digital signal is inversely proportional to the strength of the received signal: the weaker the signal, the more likely an incorrect decision will be made on whether the received signal is a "one" or a "zero". It is advantageous, therefore, to be able to monitor received signal strength (power) in a given communications channel so that the performance "health" of the channel can be readily ascertained and any problems fixed before the channel fails.
In optical communication systems, the received signal strength is the amount of optical power received at an optical-to-electrical converter or receiver, such as a PIN photodiode. One convenient way of measuring the average optical power received by a PIN photodiode is to monitor the average current (bias current) through the diode. However, due to the substantial range in optical signal power that a PIN photodiode can handle, the range in bias current can exceed the dynamic range of a monitoring circuit. Further, to adapt the monitoring circuit to tolerate the large range in PIN bias current, with the low voltage power supplies used today (typically +5 volts), the lower the headroom (the voltage drop across the diode) available to the PIN diode, thus, under high signal levels, the PIN diode may saturate, causing the system to be inoperable.
Thus, a need exists for received optical power monitoring circuit that can operate over wide power levels with low supply voltage without substantially reducing the headroom available to the optical receiver at high optical signal levels.